1. Field of the Invention
The invention relates to a method and an apparatus for preventing plugging of the outlet of a fire extinguisher due to freezing of water which is dissolved in the fire extinguishing fluid.
2. Description of the Related Art
Halon 1301 is used worldwide as the fire extinguishant for engine and cargo protection in most aircraft fire extinguishers. In installations in the cargo area of the aircraft, it is normally desirable to meter the flow of Halon 1301 out of the fire extinguisher in order to keep the cargo compartment inert for long periods of time. Generally, the metering system of the fire extinguisher includes an orifice to limit the flow of the Halon 1301. After liquid Halon 1301 passes through the orifice, the Halon expands into a gas. The expansion cools the gas to approximately −60° F. If the Halon is contaminated with water, the water may freeze in the orifice, plugging the orifice and blocking the flow of the Halon 1301.
Dryers have been placed in the line leading to the orifice in the metering system, since the use of the first metered systems in about 1980. Although the in-line dryer is supposed to remove the water from the Halon 1301 as the Halon passes from the fire extinguisher bottle through the line dryer to the outlet, the line dryers may not remove the water in a single pass. The typical in-line dryer is a refrigeration device which depends on having an unlimited time to condense water out of the Halon. However, the flow of Halon from the fire extinguisher through the in-line dryer may be too rapid to allow for condensation of sufficient quantities of water from the Halon to prevent freezing at the orifice.
Another method of preventing freezing at the outlet of the fire extinguisher is to add methanol to the Halon 1301 in the fire extinguisher bottle. The methanol forms a solution with the water and lowers the freezing point of the water. Adding methanol to the fire extinguisher bottle in an amount of about 0.5% of the weight of the Halon before filling is effective in preventing freezing of the orifice. However, some airlines do not allow methanol to be added to the fire extinguisher bottles because of concerns about contaminating their limited Halon supply with methanol. Manufacture of Halon 1301 has been banned due to ozone depletion concerns. Airlines therefore depend on being able to recycle their existing supplies of Halon 1301. Contamination of the Halon 1301 with solvents such as methanol could compromise the recycling of the used Halon 1301. There is thus a need for a means of preventing freezing of water contamination in the outlet of fire extinguishers without adding contaminating solvents.